The Average Service Unavailability Index (ASUI)^[1] is a reliability index commonly used by electric power utilities. It is calculated as

${\displaystyle {\mbox{ASUI}}={\frac {\sum {U_{i}N_{i}}}{\sum {N_{i}}\times 8760}}=1-{\mbox{ASAI}}}$

where ${\displaystyle N_{i}}$ is the number of customers and ${\displaystyle U_{i}}$ is the annual outage time (in hours) for location ${\displaystyle i}$. ASUI can be represented in relation to SAIDI (where annual SAIDI is given in hours) as

${\displaystyle {\mbox{ASUI}}={\frac {\mbox{SAIDI}}{8760}}}$

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